The atom probe (also referred to as an atom probe microscope) is a device which allows specimens to be analyzed on an atomic level. A basic version of a conventional atom probe might take the following form. A specimen mount is spaced from a detector, generally a microchannel plate and delay line anode. A specimen is situated in the specimen mount, and the charge (voltage) of the specimen holder is adapted versus the charge of the detector such that atoms on the specimen's surface ionize and “evaporate” from the specimen's surface, and travel to the detector. Generally, the voltage of the specimen is pulsed so that the pulses trigger evaporation events with the timing of the pulses, thereby allowing at least a rough determination of the time of evaporation. The specimen's atoms tend to ionize in accordance with their distance from the detector (i.e., atoms closer to the detector tend to ionize first), and thus the specimen loses atoms from its tip or apex (the area closest to the detector) first, with the tip slowly eroding as evaporation continues. Measurement of the time of flight of the ionized atoms from the specimen to the detector allows determination of the mass/charge ratio of the ions (and thus the identity of the evaporated atoms). Measurement of the location at which the ions impinge on the detector allows determination of the relative locations of the ionized atoms as they existed on the specimen. Thus, over time, one may build a three-dimensional map of the identities and locations of the constituent atoms in a specimen.
Owing to the number of atoms potentially contained in a specimen, and the time required to collect these atoms, specimens are often formed of a sample of a larger object. Such specimens are often formed by removing an elongated core from the object—often referred to as a “microtip”—which represents the structure of the sampled object throughout at least a portion of its depth. Such a microtip specimen is then usually aligned in the specimen holder with its axis extending toward the detector, so that the collected atoms demonstrate the depthwise structure of the sampled object. The rodlike structure of the microtip also beneficially concentrates the electric field of the charged specimen about its apex (its area closest to the detector), thereby enhancing evaporation from the apex.
Ionizing (evaporating) energy need not be delivered solely by means of electric fields. For example, atom probes have been developed wherein the specimen is pulsed thermally, as well as electrically, to assist with evaporation. In some prior arrangements, a laser is situated adjacent to the specimen mount to direct laser pulses at the specimen, thereby briefly heating it to induce evaporation (see, e.g., Kellogg et al., Reference 12 in the accompanying bibliography). However, such arrangements are not common because it can be difficult and time-consuming to focus the laser beam onto a microtip specimen (more particularly, onto its apex). Further, owing to this difficulty, a laser beam of relatively wide diameter is needed, but this undesirably decreases the power density of the laser (unless laser power consumption is increased, which is also undesirable). In addition, the wide beam heats a greater area of the microtip specimen, and such heat can lead to uncertainties in mass determination because the retained heat in the specimen promotes greater variation in ion evaporation times. An alternative approach proposed by Kelly et al. (Reference 1 in the accompanying bibliography) utilizes an electron beam rather than a laser and reduces heating problems, though beam focusing and specimen heating can still pose problems.
As a result, most atom probes enhance evaporation by use of other features. One such feature that may be used is a counter electrode, an electrode with a central aperture, which is situated closely spaced from the specimen between the specimen and detector (see, e.g., Miller at al., Reference 18 in the accompanying bibliography). The counter electrode is usually attractively changed with respect to the specimen so that it will enhance evaporation from the specimen, causing atoms to ionize and fly through the counter electrode's aperture toward the detector. Counter electrodes are generally used for one or more of the following purposes.
First, by situating the aperture of the counter electrode about the apex of the tip, the evaporating electrical field about the apex can be greatly enhanced, thereby allowing the use of evaporating voltage pulses of lower magnitude. Owing to equipment limitations, voltage pulses of lower magnitude usually allow faster pulsing, and thus faster evaporation rates from the specimen (and faster data acquisition). In some cases, counter electrodes are used to concentrate the evaporating field about a selected microtip on a specimen bearing multiple microtips, such that ion evaporation only occurs from the single microtip. In this situation, the counter electrode is often referred to as a “local electrode” since it allows localized evaporation (see, e.g., Kelly at al., Reference 2 in the accompanying bibliography). To achieve more focused evaporation, the local electrode generally has a much smaller aperture than a conventional counter electrode, e.g, on the order of 5-50 micrometers rather than on the order of a few millimeters.
Second, counter electrodes can be used to improve the mass resolution of the atom probe (i.e., to better calibrate measurements of ion times of flight between the specimen and detector). When atom probe voltages are pulsed, atoms tend to evaporate about the peaks of the pulses, leading to a small spread in departure times. Further, a late-departing ion may be in the region of the specimen as the voltage pulse on the specimen decays, and thus the ion may be influenced by the time-varying electrical field emitted by the specimen, leading to greater uncertainty in its true departure time (and thereby in the ion's time of flight, and in the determination of the ion's mass). However, if the counter electrode is situated sufficiently close to the specimen that a departing ion falls under the influence of the counter electrode's electric field before the specimen's voltage pulse significantly decays, the ion's flight will largely be decoupled from the time-varying field, thereby reducing its effect.
Third, counter electrodes are sometimes used to shield the specimen from components in the flight path that might affect the electric fields near the specimen apex. As an example, if an atom probe microscope has a movable detector, the field on the specimen may be increased if the detector is moved closer, thereby enhancing the possibility of ion evaporation at unwanted times and complicating operation. However, the counter electrode, being situated between the specimen and the detector, can partially isolate the specimen from the detector and reduce the influence of the detector's field.